1. Field of the Invention
The present invention relates to a semiconductor laser device formed mainly of InGaAlP and used for a short-wavelength band of 580 to 680 .mu.m.
2. Description of the Related Art
Among the semiconductor laser devices formed of the elements belonging to the III to V groups of the periodic table, a double hetero structure semiconductor laser device whose activation layer and clad layer are formed of a four-element mixed crystal of InGaAlP produces the shortest oscillation wavelength. Therefore, in comparison with a currently-adopted GaAlAs-based semiconductor laser device whose wavelength band is between 780 and 830 nm, it is suitable for use in an optical information processor using an optical disk, a laser printer, or a light source for plastic fibers. Moreover, it can be used in a bar code reader, in place of a He-Ne gas ion laser device having its peak at 633.8 nm, or in an optical measurement controller.
In general, an InGaAlP-based semiconductor laser device is manufactured by use of a GaAs substrate. A double hereto laser of InGaAlP is formed on the surface of the GaAs substrate in a manner to achieve lattice matching with the GaAs substrate.
The four-element mixed crystal of InGaAlP is normally expressed as In.sub.l-y (Ga.sub.l-x Al.sub.x).sub.y P. If y in this formula is nearly equal to 0.5, the mixed crystal achieves lattice matching with respect to the GaAs substrate on the order of less than .+-.1.times.10.sup.-3, even if the value of x (i.e., the mixing rate) is arbitrary.
In the case where y=0.5, the band gap energy Eg of the mixed crystal, i.e., In.sub.0.5 (Ga.sub.1-x Al.sub.x).sub.0.5 P, takes of the value of 1.91 eV to 2.25 eV, provided that x=0.about.1.
M. Kazumura et al., Jpn. Appl. PHYS., 22 654 (1983) shows the relationship between the lattice constant of the mixed crystal of InGaAlP and the energy gap (Eg), with the lattice constant and the energy gap being plotted against the axis of ordinates and the axis of abscissas, respectively.
According to the document, when Eg=2.17 eV, the transition regions for direct transition and indirect transition correspond to the case where the mixing rate x is equal to 0.7. Therefore, a mixing rate x of 0.about.0.2 (.lambda.=580.about.680 nm) is normally used for an activation layer, and a mixing rate x of not less than 0.4 (x.gtoreq.0.4) is normally used for a clad layer. The crystal growth of the activation layer and clad layer is performed by use of the MOCVD (metal organic chemical vapor deposition) method, the MBE (molecular beam epitaxy) method, or the like.
FIG. 1 shows a cross section of the main portion of a conventional InGaAlP semiconductor laser device obtained by use of the MOCVD method. This semiconductor laser device is of a gain waveguide type which has a so-called inner stripe (IS) structure, wherein a current blocking layer formed of GaAs is located on the upper side of a double hetero layer formed of InGaAlP.
The above conventional semiconductor laser device is manufactured as follows.
First of all, the following layers are formed on the surface of n-type GaAs substrate 50 by use of the MOCVD method: lower clad layer 51 formed on n-type In.sub.0.5 (Ga.sub.0.5 Al.sub.0.5).sub.0.5 P; undoped activation layer 52 formed of In.sub.0.5 Ga.sub.0.5 P; upper of clad layer 53 formed of p-type In.sub.0.5 (Ga.sub.0.5 Al0.5).sub.0.5 P; current supply-facilitating layer 54 formed of p-type In.sub.0.5 Ga.sub.0.5 P; and current-blocking layer 55 formed of n-type GaAs. The thicknesses of the respective layers are determined as follows: 1 .mu.m for lower clad layer 51; 0.08.about.0.1 .mu.m for undoped activation layer 52; 1 .mu.m for upper clad layer 53; 0.05 .mu.m for current supply-facilitating layer 54; and 0.5 .mu.m for current-blocking layer 55.
As dopants for determining the conductivity type of each layer, Zn and one of Si and Se are used.
The carrier concentration of each layer is as follows: 51.times.10.sup.18 cm.sup.-3 for lower clad layer 51; less than 1.times.10.sup.16 cm.sup.-3 for undoped activation layer 52; 2.times.10.sup.17 cm.sup.-3 for upper clad layer 53; (2.about.8).times.10.sup.18 cm.sup.-3 for current supply-facilitating layer 54; and (1.about.2).times.10.sup.18 cm.sup.-3 for current-blocking layer 55.
Next, mesa stripe groove 56 is formed in current-blocking layer 55. Thereafter, constant layer 57 formed of p-type GaAs is deposited on the resultant structure by use of the MOCVD method such that contact layer 57 has a thickness of 1 to 3 .mu.m and a carrier concentration of 1.times.10.sup.19 cm.sup.-3.
Since there is a large energy difference between the valence bands of InGaAlP and GaAs, current supply-facilitating layer 54 serves to suppress formation of a high-resistance energy barrier at the hereto interface between the p-type regions.
After crystal growth is performed with respect to each layer in the above manner, the surface of GaAs substrate 50 is lapped and specularly polished, while simultaneously adjusting the thickness of GaAs substrate 50 to about 80 .mu.m. Then, ohmic electrodes (not shown), such as Au/Zn (p side) and Au/Ge (n side) are arranged on the exposed surface portions of contact layer 57 and GaAs substrate 50, respectively. Thereafter, GaAs substrate 50 is divided into chips of predetermined shapes, and the p-side of the chip is mounted on a heat sink.
The characteristics of the InGaAlP semiconductor laser device fabricated in the above fashion depend greatly on the p-type dopant doped into the InGaAlP. In general, Zn is most suitable for use as a p-type dopant, in light of controllability and stability. More specifically, in comparison with the case where GaAs or GaAlAs is used, a small amount of Zn is taken into the crystal at the time of vapor phase growth process, such as the vapor phase growth process of the MOCVD method. Further, Zn is hard to activate as a carrier in the crystal. These tendencies will become more marked with an increase in the Al mixing rate x.
FIG. 2 shows how the specific resistance is dependent on the mixing rate x in the case where n-type and p-type impurities are doped into the In.sub.0.5 (Ga.sub.l-x Al.sub.x).sub.0.5 deposited on the GaAs substrate by use of the MOCVD crystal growth process. As can be understood from FIG. 2, the p-type InGaAlP has resistance about ten times as high as that of the n-type InGaAlP, and the resistance of the p-type In GaAlP rapidly increases in the range where x.gtoreq.0.5 (in which range, the p-type InGaAlP is used as a p-type clad layer of a semiconductor laser device). Therefore, FIG. 2 shows how the specific resistance of the n-type or p-type InGaAlP is dependent on Al. The source of this technical information is I. Hino et al., Jpn. Appl. Phys., 23, No. 9, l 746 (1984).
Since, as mentioned above, Zn is hard to activate in the crystal, the crystal contain a large amount of Zn which does not function as a carrier. Further, the amount of Zn which taken into the crystal increases with an increase in the mixing rate x. Therefore, most of the Zn becomes defective after taken into the crystal, with the result that both the optical and thermal characteristics of the crystal are adversely affected.
In the above InGaAlP-based semiconductor laser device, the p-type upper clad layer has a decisive effect on the characteristics of the laser device. More specifically, since the upper clad layer has high resistance, the series resistance of the elements is high, necessitating application of high driving voltage. In addition, if the resistance of the upper clad layer is higher than those of the other layers, the region in which current flows expands laterally just below the current-blocking layer. Accordingly, the current introduction into the activation layer requires a large area and cannot be performed without causing a certain degree of loss. As a result, the threshold current and the driving current increase. It should be also noted that the thermal resistance of InGaAlP is higher than those of GaAs and GaAlAs. (The thermal resistance becomes higher with an increase in the Al mixing rate x, and is maximum when Al/Ga=1.) However, since the p-type upper clad layer has a low carrier concentration, its thermal resistance is inevitably higher than that of the n-type lower clad layer.
For the reasons stated above, the thermal radiation of the light-generating region is not good, and therefore the laser characteristics are markedly affected at a high temperature. In addition, since the p-type upper clad layer contains defects caused by the large amount of Zn taken therein, it is likely that the characteristics of the upper clad layer will deteriorate when exposed to light or heat. An element whose characteristics have been deteriorated was actually examined, and the examination showed that the deterioration was due to the defects (such as DLD and DSD) caused in the p-type upper clad layer.
The problems noted above become more serious with an increase in the Al mixing rate x of the p-type upper clad layer. However, if the Al mixing rate is decreased, the carriers may be confined to a limited region, and the light may be liable to leak. Therefore, it becomes necessary to thicken the upper clad layer. In this way, the problems regarding the p-type clad layer cannot be solved even if the mixing rate x is varied.